1. Technical Field
The present disclosure relates to an acoustic attenuator for use in downhole applications. More particularly, the present disclosure relates to a low frequency downhole acoustic attenuator configured to attenuate acoustic signals at frequencies below 5 kHz and preferably in a low frequency bandwidth extending from about 300 Hz to about 2 kHz.
2. Description of Related Art
Geologists and geophysicists are often interested in the characteristics of subsurface earth formations encountered by a drill bit as it drills a well for the production of hydrocarbons from the earth. Such information is useful in determining the correctness of the geophysical data used to choose the drilling location and in choosing subsequent drilling locations. In horizontal drilling, such information can also be useful in determining the location of the drill bit and the direction that drilling should follow. It should be readily appreciated by those skilled in the art that the foregoing is but a rudimentary description of some of the ways in which information regarding the characteristics of a subsurface earth formation would be useful to geologists, geophysicists and/or others involved in the exploration of the subsurface earth formation. It is, however, a sufficiently detailed description for the purposes disclosed herein.
Information related to the characteristics of a subsurface earth formation can be derived in a number of ways. For example, cuttings from the mud returned from the drill bit location can be analyzed or a core can be bored along the entire length of the borehole. Alternatively, the drill bit can be withdrawn from the borehole and a wireline logging tool can be lowered into the borehole to take measurements. Other approaches, commonly referred to as either measurement-while-drilling (MWD) or logging-while-drilling (LWD) techniques, involve the use of tools which make measurements in the borehole while the drill bit is working. Of the aforementioned techniques, MWD and LWD are generally preferred because, by employing such techniques, information related to the subsurface earth formation may be acquired quicker and at lower cost. Currently, however, the bandwidth available for downhole telemetry systems, for example, mud pulse or electromagnetic (EM) telemetry systems, is insufficient for transmitting essential MWD or LWD data in real time. As a result, data acquired by a MWD or LWD tool is often stored in a local memory in the tool and not retrieved from the local memory until after the tool has been removed from the borehole. As a result, data acquired by a MWD or LWD tool is not always available for analysis in real time.
The transmission of acoustic telemetry data through the drill pipe itself has been periodically contemplated. Acoustic telemetry systems which transmit acoustic data through a drill pipe have been able to transmit data at rates exceeding 50 bits-per-second (bps). A problematic issue related to the transmission of acoustic data through a drill pipe while drilling operations are being conducted is that on-going drilling operations tend to generate wide bandwidth noise which contaminates the acoustic data transmission channel through which acoustic data is being transferred. One source of such noise is the drill bit typically located at the end of the drill pipe. Another is the top drive or other mechanical equipment located at the surface and coupled to the drill pipe. Regardless of the source thereof, such noise tends to reduce the signal-to-noise (SNR) ratio within the acoustic data transmission channel. As a result, the rate at which acoustic data may be transmitted through the acoustic data transmission channel is reduced.
It should be appreciated that, by incorporating one or more devices capable of absorbing low frequency acoustical energy into a drill string, the amount of noise entering an acoustic data transmission channel of the drill string would be reduced. As a result, data acquired by a downhole data transceiver and transmitted to the surface via the acoustic data transmission channel is less likely to be contaminated by extraneous noise.